1. Field of the Invention
The present invention relates to a martensitic stainless steel suitable for use as a material of members which operate at high speed in water, such as foils and struts of a high speed vessel, e.g., a hydrofoil, runner of a water turbine, and so forth. More particularly, the present invention is concerned with a martensitic steel which is superior in strength, corrosion resistance, erosion resistance, fatigue properties and weldability, as well as an advantageous method for producing such a steel.
2. Description of the Related Art
In recent years, speed of vessel is becoming higher, as well as operation speed of high-speed rotation members such as a water turbine runner. This has given a rise to the demand for higher corrosion and erosion resistances of the steel.
In particular, foils and struts of high-speed vessel such as a hydrofoil are required to have high strength partly because they are required to bear the weights of cargo, passengers and the hull and partly because they must be constructed to have a reduced weight to reduce the total weight. In addition, the whole or part of the load is applied repeatedly to the struts and foils as a result of rolling and pitching of the vessel. The frequency of application of such a repetitious load is not so high as that in the case of a propeller or a water turbine. In addition, the foils and struts are used in sea water. Therefore, the steel used as a material for such a hydrofoil is required to have high strength against low cyclic frequency fatigue load in sea water, as well as high resistances to corrosion and erosion which are caused by sea water which attacks the foils and struts at high relative velocity.
In order to cope with these demands, Ni-containing steels containing 13 wt % of Cr and 3 to 5 wt % of Ni have been used. As disclosed in Japanese Patent Publication No. 42-16870, this type of steel has been produced by cooling the steel after full-austenization, followed by a tempering at 550.degree. to 650.degree. C. so as to allow formation of 15 to 40 wt % of retained austenite. This type of steel exhibits a proof strength of 60 to 70 kgf/mm.sup.2, as well as good toughness.
Martensitic stainless steels generally exhibit inferior weldability and workability as compared with austenitic stainless steel. A steel ASTM CA6NM (13Cr-4Ni), has been developed as cast steel material of having section structure. However, cast steel products in general exhibit quite inferior resistance to erosion because of casting defect. Furthermore, existence of internal casting defect impairs the soundness of the whole product. In order to obviate this problem, Japanese Patent Laid-Open Publication No. 1-127620 discloses a method of producing a martensitic stainless steel employing a hot rolling step. More specifically, in this method, a cast martensitic stainless steel is subjected to hot rolling so as to extinguish any casting defect, thereby greatly reducing any degradation in the resistance to erosion and in fatigue strength. This steel, however, exhibits a proof strength of 60 to 70 kgf/mm.sup.2 at the greatest, which is still unsatisfactory.
The current demand for higher speed of high-speed boats requires the weight of the foils and struts to be reduced. There also exists a demand for higher operational speed of rotary machines such as a runner of a water turbine. Under these circumstances, there is an increasing demand for high-strength martensitic stainless steel having a proof strength of 80 kgf/mm.sup.2 or greater. In general, steels having greater strength exhibit inferior weldability, as well as lower fatigue strength and resistance to erosion. Therefore, it has been difficult to obtain a high strength without being accompanied by impairment in the characteristics such as weldability, resistances to erosion and corrosion and fatigue strength.
A steel called 17-4PH steel has been known as an example of a high-strength stainless steel having a proof strength of 80 kgf/mm.sup.2 or greater. In the production of this steel, precipitation hardening heat treatment is necessary to precipitate carbides and Cu so as to attain a high proof strength. When this precipitation hardening type high-strength steel is subjected to a welding, however, the precipitates are dissolved again in the weld region due to heat applied during welding, resulting in a reduction in the strength. In order to obtain a desired strength, therefore, the welded structure has to be subjected again to a precipitation hardening treatment. Thus, the known steels of the type described require cumbersome heat treatment repeatedly after welding. Further, when the welded structure is large in size, for applying such a heat treatment after welding, a large-size heat-treating furnace is required.
Japanese Patent Laid-Open Publication No. 62-124218 discloses a method of producing a high-strength stainless steel in which contents of alloy elements such as Ni and Mn are adjusted to keep the Ms point near the level of the room temperature, and annealing is conducted at a specific temperature range for a specific time, thereby attaining a high workability and high weld-softening resistance. Japanese Patent Laid-Open Publication No. 62-124218, however, does not mention at all any measure for improving fatigue properties of the steel in corrosive or erosive condition. In addition, use of large amounts of alloy elements is uneconomical, and there is an increasing demand for a high-strength stainless steel which do not require addition of large quantity of alloy elements.
Japanese Patent Publication No. 61-23259 discloses stainless steel which exhibits superior ductility at the weld region and corrosion resistance due to formation of a massive martensitic structure at the weld region by virtue of adjustment of the contents of elements such as C, N, Cr and Ni. Japanese Patent Publication No. 61-23259, however, does not mention at all fatigue characteristic of the steel in corrosive and erosive condition, as well as the strength level of the material, although a description is made as to corrosion resistance, bending characteristic and toughness of the weld region formed by the use of a 410 Nb welding rod. The adjustment of alloy-element contents, particularly the amount of Al, in Japanese Patent Publication No. 61-23259 differs from the present invention in which the adjustment of these contents is intended to improve the fatigue strength of the material in sea water.
Japanese Patent Publication Nos. 2-243739 and 2-243740 disclose methods of producing steel suitable for use in oil and gas wells, through specific adjustment of elements such as C, N, Cr, Ni, Nb and V. Steels produced by the methods disclosed in these publications exhibit improved corrosion resistance under corrosive conditions containing carbon dioxide gas, hydrogen sulfide and chloride ions. These publications, however, do not mention at all improvement in fatigue properties of the steels when used in corrosive conditions to which the present invention is directed. In particular, these publications do not at all mention the effect of addition of Cu which is proposed by the present invention and which produces a significant effect in improving fatigue properties in sea water. Furthermore, these publications fail to mention proof strength which is a significant factor in structural designs. Thus, none of the prior art references mentioned above provide any idea for attaining an improvement in low cyclic frequency fatigue properties in sea water to which the present invention pertains.